Zhang, who attends school in the United States, gained his spot in the 156-strong field at the Olympic Club when Englishman Casey pulled out because of a shoulder injury.
American Tadd Fujikawa was previously the youngest player since World War Two at a US Open - aged 15 years, five months and seven days during the 2006 edition at Winged Foot.
US Open records are not fully comprehensive before 1945, but United States Golf Association officials told Reuters Zhang was believed to be the youngest player ever at the championship.
Casey, who dislocated his shoulder while snowboarding in January, has competed in only seven events worldwide this year, missing the cut in five of them.
"You're always disappointed to miss time due to injury but it's especially true when you have to withdraw from the US Open," Casey said.
"I've been trying to play my way back from my shoulder injury, and I feel like I probably came back too soon originally and it's affecting my ability right now to be ready for the test of a Major.
"I do feel like I'm very nearly ready but not quite ready for this week," added Casey, whose world ranking has dropped from 20th to 58th since he suffered the injury.
Earlier on Monday, three-times US PGA Tour champion Snedeker withdrew from the year's second major due to a rib injury.
The 31-year-old Snedeker, who clinched his most recent title on the US circuit at the Farmers Insurance Open in January, will be replaced by American amateur Jordan Spieth.
A twice former US junior amateur champion, Spieth will tee off with compatriots Bill Haas and Nick Watney in Thursday's opening round.